Added decoding/trace writing tests for sampling extra events

The goal of this PR is to allow support to sample other events from perf, e.g.
`branch-misses`, `cache-misses`. Although this is a nice feature, the goal is
also to already have support for processing these events through sampling, so
that when we have PEBS-via-PT available, we can also have these events in the
processor trace stream.

Now the user can pass `-events` along with a comma separated list of either
names like `branch-misses` or sexps which allow the user to control the event
period. These will then show up on separate tracks along with the actual trace
as arrows representing every sample.

I also drafted a [new wiki
page](https://github.com/Lamoreauxaj/magic-trace/wiki/Sampling-extra-events) and
referenced in the help docs for `-events` which would need to be updated along
with this PR.

This commit solely adds new tests which include both branch-misses and
cache-misses as well as the Intel PT stream or a stacktrace sampled stream.

Signed-off-by: Aaron Lamoreaux <alamoreaux@janestreet.com>

test/demo_extra_events.mli

@@ -0,0 +1 @@
+(*_ This signature deliberately left empty. *)

test/perf_script.ml

@@ -26,11 +26,13 @@ let run ?(debug = false) ?ocaml_exception_info ~trace_scope file =
     ;;
 
     let write_duration_begin ~args:_ ~thread:_ ~name ~time : unit =
-      printf "-> %8s BEGIN %s\n" (Time_ns.Span.to_string_hum time) name
+      if not String.(name = "branch-misses" || name = "cache-misses")
+      then printf "-> %8s BEGIN %s\n" (Time_ns.Span.to_string_hum time) name
     ;;
 
     let write_duration_end ~args:_ ~thread:_ ~name ~time : unit =
-      printf "-> %8s END   %s\n" (Time_ns.Span.to_string_hum time) name
+      if not String.(name = "branch-misses" || name = "cache-misses")
+      then printf "-> %8s END   %s\n" (Time_ns.Span.to_string_hum time) name
     ;;
 
     let write_duration_complete ~args ~thread ~name ~time ~time_end : unit =
@@ -86,17 +88,18 @@ let run ?(debug = false) ?ocaml_exception_info ~trace_scope file =
       in
       List.iter split_lines ~f:(fun lines ->
           let event =
-            Perf_decode.For_testing.to_event lines
-            |> Option.value_exn
-            |> adjust_event_time
+            Perf_decode.For_testing.to_event lines |> Option.map ~f:adjust_event_time
           in
-          if should_print_perf_line event
-          then (
-            match lines with
-            | [ line ] -> printf "%s\n" line
-            | lines -> print_s [%message (lines : string list)]);
-          let event = Event.With_write_info.create ~should_write:true event in
-          Trace_writer.write_event trace_writer event);
+          match event with
+          | Some event ->
+            if should_print_perf_line event
+            then (
+              match lines with
+              | [ line ] -> printf "%s\n" line
+              | lines -> print_s [%message (lines : string list)]);
+            let event = Event.With_write_info.create ~should_write:true event in
+            Trace_writer.write_event trace_writer event
+          | None -> ());
       printf "END\n";
       Trace_writer.end_of_trace trace_writer)
 ;;